1. Field of the Invention
The present invention relates to a composition and method for improving the storage stability of polymer modified asphalts.
2. Discussion of Related Art
Asphalt is a bituminous material resulting from the distillation of crude oil. Typically, asphalt is derived from the bottoms of a vacuum distillation tower and has an atmospheric boiling point of at least 380.degree. C. Because it is hydrophobic and has good adhesiveness and weatherability, asphalt has been used widely as a binder in paving materials and as a coating for roofing shingles.
Frequently, polymeric materials have been added to asphalt to enhance its rheological properties, i.e., to improve its creep resistance above about 20.degree. C. Polymer modified asphalts must also have good phase compatibility between the asphalt and polymer, and be storage stable at high temperatures for ease of handling and application. Compatibility between the polymer and asphalt is very important to ensure that the properties of both are transferred to the finished product for good long term performance. Poor storage stability will render the polymer modified asphalt unsuitable for use in paving binder applications, roofing applications, and other industrial specialty products. Accordingly, various methods have been suggested for maintaining storage stability.
One method requires continuous mixing of the asphalt/polymer mixture to avoid phase separation (See, for example, U.S. Pat. Nos. 4,240,946 and 4,314,921, which require high shear mixing to obtain physical dispersion of a polyolefin in bitumen. See also Transportation and Road Research Laboratory Report 1101 by J. H. Denning et. al., Highways and Structures Department, Crowthorne, Berkshire, England (1983)). Another method discloses adding one or more dispersants to a polyethylene modified asphalt to avoid phase separation (See Jew et al., Journal of Applied Polymer Science, 31, pp.2685-2704 (1986)).
In yet another method, the composition of the asphalt is tailored to ensure compatibility with the polymer used or the polymer is selected to be compatible with the asphalt (See U.S. Serial Nos. 073,813 filed June 15, 1987 and 134,954 filed Dec. 18, 1987).
In still another method, polymer modified asphalt is stabilized by the addition of an acid after the polymer has been added to the asphalt (See, for example, German patent DE 22 55 173 C3 which discloses stabilizing mixtures of asphalt and styrenic thermoplastic elastomers (styrene-butadiene-styrene) by adding small amounts of 85% o-phosphoric acid or 36% hydrochloric acid to the asphalt/SBS mixture).
However, none of these methods, alone or in combination, teach or suggest the specific method for improving the storage stability of polymer modified asphalts described hereinafter.